In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, the so-called new radio (NR) access technology being considered for fifth generation (5G) telecommunications system could utilize so-called massive multiple-input multiple output (MIMO) systems for transmission and reception. To reach the full potential of massive MIMO, full control of amplitude and phase of each antenna element of the transmitters and receivers in the communications system could be needed.
Furthermore, channel state information could be needed at the transmitter to be able to set the amplitude and phase per antenna element in an optimal way. Therefore, there could be a need to utilize channel reciprocity since the signaling overhead to estimate the radio propagation channel between a large number of antenna element pairs (one at the transmitter and the other at the receiver) by means of feedback could be overwhelming.
When per-antenna element channel estimates are available at the transmitter, precoding mechanisms for both single- and multi-user MIMO can be used. For example, in a rich scattering radio propagation channel where conventional beamforming gives little or no array gain, maximum ratio transmission (MRT) can in theory give full array gain by focusing energy to a single point in space rather than in a single direction. Moreover, spatial nulling towards co-scheduled users when using multi-user MIMO transmission can be used to mitigate inter-user interference.
The antenna elements in both the transmitter and the receiver are part of antenna arrays. Existing antenna arrays comprising comparatively many antenna elements are commonly based on analog or hybrid analog/digital beamforming architectures. This has been motivated by the high cost, complexity and power consumption that have been associated with a fully digital architecture. However, recent advances in digital technology have made a fully digital implementation a viable, or sometimes even preferred, architecture for the antenna arrays.
Even though a fully digital antenna array with digital control of each individual antenna element and arranged for massive MIMO transmission and reception is realistic to implement in a cost-effective way, the comparatively large amount of data massive MIMO transmission and reception generate poses challenges, both in terms of digital processing capacity and data interface requirements.
Therefore, it is currently not realistic to assume that a full digital baseband chain will be connected to each individual antenna element. It is therefore envisioned that data reduction could be performed before the data from the antenna array is processed at baseband. One way to achieve this is to apply a beam space transformation and then select a few beams to be further processed at baseband. One issue with such a beam space data reduction is that it is no longer possible to obtain channel estimates per antenna element since data for only a few beams are available at baseband.
Hence, there is still a need for improved channel estimation.